Bobcat M4903 (49-03) Fault Code: Rear Light Relay Error Off
Also shown on the panel as 49-03
Rear Light Relay Error Off · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
M4903 (panel code 49-03) sets when the gateway controller commands the rear lights on but does not detect voltage for 5 seconds on the rear light relay feedback wire (3440 or 6230 depending on model). It points to a rear lighting circuit problem, usually a blown fuse, bad relay, damaged wire, or a corroded connector, not a driving safety-critical failure but one that affects night visibility and compliance.
Medium severity. M4903 does not disable machine functions or threaten engine, hydraulic, or structural damage. It should still be diagnosed promptly because working without rear lights is a real visibility and safety hazard during low-light operation, transport, or backing up.
What does Bobcat error code M4903 mean?
M4903 is a Bobcat gateway controller fault tied to the rear light relay feedback circuit. The gateway controller commands the rear lights on and then checks a feedback wire (labeled 3440 on some machines, 6230 on others) to confirm voltage actually reached the rear lights. If that feedback voltage isn't detected for 5 seconds after the command, the controller logs M4903 and reports a rear light relay error.
This is strictly a monitoring and feedback issue on the rear lighting circuit. It doesn't affect propulsion, hydraulics, or engine operation. The practical effect is that your rear work lights or rear lighting may not be functioning even though the switch or control has commanded them on.
Because the fault is about a missing confirmation signal, the actual failure could be anywhere in the circuit: the fuse feeding the circuit, the relay itself, the feedback wire, the supply wires, or the connectors between the gateway controller and the tailgate harness.
What triggers a Bobcat M4903 code?
The code sets when the key switch is in the run/enter position, rear lights are enabled/commanded on, and the gateway controller fails to detect voltage on the rear light relay feedback wire (3440 or 6230, depending on model) for 5 seconds.
Common causes of M4903
- FRC1 fuse (listed as fuse 3 on some models, fuse 9 on others) blown or failing.
- Rear light relay feedback wire (3440 or 6230 depending on model) shorted to ground or open.
- Rear work lights wire 6200 shorted to ground.
- Supply wires 1320 or 1340 open.
- Faulty rear light relay.
- Faulty gateway controller.
- Moisture, corrosion, or pushed-back pins in the gateway controller connectors, rear light relay connectors, tailgate connector, tailgate harness, or mainframe harness.
- Intermittent fault: if the code isn't active at the time of testing, the cause may not be identifiable in that session; likely culprits are still loose connections, corrosion, pushed-back pins, the controller, or the wire harness.
How to troubleshoot Bobcat M4903: first checks
- Check FRC1 fuse (fuse 3 or fuse 9 depending on model) for continuity; replace if blown.
- Inspect the rear light relay and its connector for damage, corrosion, or looseness; swap with a known-good relay if available to test.
- With the key on and rear lights commanded on, check for voltage on the feedback wire (3440 or 6230) at the gateway controller connector.
- Inspect wire 6200 (rear work lights) for chafing or shorts to ground, especially near the tailgate and mainframe harness routing.
- Check supply wires 1320 and 1340 for continuity, open circuits, or damage.
- Inspect the tailgate connector, tailgate harness, and mainframe harness connectors for moisture intrusion, corrosion, or pushed-back pins.
- If the code was intermittent and not present during testing, wiggle-test the harness and connectors while monitoring for the fault to reappear, since the root cause may not show up on a static check.
How the code clears
Once the repair is complete, cycle the key power (turn the key off, then back to run/enter) to clear the code. No separate reset procedure is listed for this code.
Affected models and serial ranges
M4903 appears in our records across 16 Bobcat models. Match your machine by model and serial number.
| Model | Serial ranges |
|---|---|
| S510 | SN A3NJ11001-99999, SN A3NK11001-99999, SN ATZC11001-ATZC99999 |
| S530 | SN A7TV11001-99999, SN ATZD11001-99999 |
| S550 | SN A3NK11001-A3NL99999, SN A3NM11001-99999 |
| S570 | SN A7U711001-799999, SN A7U811001-899999 |
| S590 | SN ANMN11001-99999, SN ANMP11001-99999 |
| S630 | SN A3NT12370-99999, SN A3NU11001-11111, SN A3NU11112-99999 |
| S650 | SN A3NV11001-13098, SN A3NV13099-99999, SN A3NW11001-11248, SN A3NW11249-99999 |
| S750 | SN A3P211001-299999 |
| S770 | SN A3P411001-99999 |
| T550 | SN A7UJ11001-AJZV12276 |
| T590 | SN ALJU11001-999999, SN B37811001-999999, SN B3Z711001-999999 |
| T630 | SN A7PU11001-11663, SN A7PU11664-99999 |
| T650 | SN A3P012214-099999, SN A3P111242-199999 |
| T750 | SN ANKA11001-A99999 |
| T770 | SN A3P811001-899999, SN A3P911001-999999 |
| T870 | SN A3PG11001-99999, SN A3PH11001-99999 |
Frequently asked questions
What does Bobcat code M4903 mean?
It means the gateway controller commanded the rear lights on but didn't detect voltage on the rear light relay feedback wire (3440 or 6230, depending on model) within 5 seconds. It's a rear lighting circuit feedback fault, not an engine or hydraulic issue.
Is M4903 dangerous to keep operating with?
It won't damage the machine, but if your rear lights aren't actually working, that's a real visibility hazard when operating in low light, backing up, or on public roads. Diagnose it before running in the dark.
What's the most common cause of M4903?
A blown fuse (FRC1 fuse 3 or fuse 9 depending on model) or a bad rear light relay are common starting points, along with corroded or loose connectors around the tailgate and mainframe harness.
How do I clear M4903 after fixing it?
Cycle the key power off and back to run/enter. No other reset procedure is listed for this code.
Why does M4903 come and go instead of staying on?
Intermittent triggering usually points to a loose connection, corrosion, or a pushed-back pin somewhere in the gateway controller, relay, or tailgate/mainframe harness connectors. If the code isn't active when you test, you may need to wiggle-test the harness to catch it.
Which Bobcat models use code M4903?
It applies across S510, S530, S550, S570, S590, S630, S650, S750, S770, T550, T590, T630, T650, T750, and T770/T870 models, though the specific feedback wire number (3440 vs 6230) and fuse number (3 vs 9) can differ by model.
Does M4903 affect anything besides the rear lights?
No. This code is isolated to the rear light relay feedback circuit monitored by the gateway controller. It doesn't affect drive, hydraulics, or engine systems.